1. Field of the Invention
The present invention relates to a plastic molded gear, a gear train using the plastic molded gear, and an intermittent rotation transmission device, such as a paper feeding mechanism for an ink jet printer, which uses the plastic molded gear.
2. Description of the Prior Art
In comparison with metal gears, plastic molded gears are light and low noise gears, and can be used without any lubricants since they have self-lubricating properties. In addition, the mass-productivity of plastic molded gears is higher than that of metal gears, so that the production costs for plastic molded gears can be lower than those of metal gears. For that reason, in recent years, plastic molded gears are widely used for gear trains for ink jet printers, and gear trains for automotive power transmission parts and precision instruments.
Of such plastic molded gears, in plastic molded gears used for intermittent rotation transmission devices for transmitting power while frequently repeating rotation and stopping, one of teeth meshing with each other during stopping and starting collides with the other tooth, so that there are discrepancies that teeth knocking noises are produced.
Therefore, there are proposed various plastic molded gears which are devised so as to prevent teeth knocking noises during power transmission.
As a first conventional example, a gear 50 shown in FIGS. 20A and 20B has fillet portions 52, which are formed on both tooth flanks on one end side in facewidth directions of each tooth 51 and which are designed to contact a companion gear meshing with the gear 50 to be deformed to absorb shocks when the gear 50 meshes with the companion gear (see Japanese Utility Model Laid-Open No. 55-100745).
As a second conventional example, a gear 53 shown in FIGS. 21A and 21B has slits 56, each of which is formed on the side of a tooth flank 55 meshing with a companion gear 54 so as to extend along the tooth flank 55 and so as to pass through each tooth in facewidth directions, and each of which allows the tooth flank 55 meshing with the companion gear 54 to be elastically deformed to absorb shocks when the tooth flank 55 meshes with the companion gear 54 (see Japanese Utility Model Laid-Open No. 58-127246).
As a third conventional example, a gear 57 shown in FIG. 22 has through holes 60, each of which passes through each tooth 58 in facewidth directions, and each of which allows the whole tooth 58 to be easily elastically deformed to absorb shocks when the gear 57 meshes with a companion gear (see Japanese Utility Model Laid-Open No. 55-98849).
As a fourth conventional example, a gear 61 shown in FIG. 23 has slits 63, each of which is formed in each tooth 62 so as to extend in facewidth directions over the entire facewidth and so as to extend from a tooth crest 64 to pass through a bottom land 65, and each of which allows the tooth 62 to be easily elastically deformed to shocks when the gear 61 meshes with a companion gear (see Japanese Utility Model Laid-Open No. 55-98850).
As a fifth conventional example, a gear 66 shown in FIG. 24 has visco-elastic bodies 70, each of which protrudes from both tooth flanks 68 of each tooth 67 in a central portion in facewidth directions, and each of which is designed to contact a tooth flank of a companion gear to absorb shocks when the gear 66 meshes with the companion gear (see Japanese Patent Laid-Open No. 2001-221322).
As a sixth conventional example, a gear 71 shown in FIG. 25 and proposed by the inventor of the present application has cavity portions 73, which are formed in both end portions in facewidth directions of each tooth 72, and elastically deformable expanded protruding portions 74 which are formed on both end portions in facewidth directions of each tooth 72 by the difference in shrinkage after injection molding between both end portions having the cavity portions 73 in facewidth directions and other portions. The expanded protruding portions 74 are designed to be elastically deformed to absorb shocks when the gear 71 meshes with a companion gear (see Japanese Patent Laid-Open No. 2003-90412).
However, in the first conventional example, the fillet portions 52 are formed on only one end side in facewidth directions, and thin walled portions and cavity portions capable of absorbing deformation of the fillet portions 52 are not formed. Therefore, the surface pressure applied on the fillet portions 52 is too large, so that the fillet portions 52 are early worn. Thus, there is the possibility that, when the gear meshes with a companion gear, it is not possible to obtain shock absorbing effects for a long period of time, so that it is not possible to stably obtain teeth knocking noise absorbing effects for a long period of time.
In the second conventional example, since the tooth flank 55 meshing with a companion gear during power transmission is elastically deformed over the whole region in facewidth directions, the amount of elastic deformation varies in accordance with load during power transmission, so that the angle of rotation per tooth is easy to vary. Therefore, it is difficult for the gear 53 to be used for a gear train for precisely transmitting rotation.
In the third and fourth conventional examples, since the whole teeth 58 and 62 are easily deformed, the amount of elastic deformation of the teeth 58 and 62 varies in accordance with load during power transmission similar to the second conventional example, so that the angle of rotation per tooth is easy to vary. Therefore, it is difficult for the gears 57 and 61 to be used for a gear train for precisely transmitting rotation.
In the fifth conventional example, although the visco-elastic bodies 70 protruding from the tooth flanks 68 are compressed to be deformed by load during power transmission, the amount of deformation thereof varies in accordance with the load during power transmission. Therefore, similar to the second through fourth conventional examples, it is difficult for the gear 66 to be used for a gear train for precisely transmitting rotation.
In the sixth conventional example, if the expanded protruding portions 74 are elastically deformed, the tooth flank 75 except for the expanded protruding portions 74 contacts the tooth flank of a companion gear to transmit power. Therefore, it is possible to more precisely transmit rotation than the gear in the second through fifth conventional examples. In addition, since the expanded protruding portions 74 are formed on both end portions in facewidth directions, it is possible to decrease the surface pressure when the gear 71 meshes with the companion gear, and it is difficult to cause the early wearing of the teeth 72 unlike the first conventional example. However, since the expanded protruding portions 74 are formed on the basis of the difference in shrinkage after injection molding, the amount of expanded protrusion is small, so that it is not possible to cause the expanded protruding portions 74 to protrude from the tooth flanks 75 to such an extent that it is possible to sufficiently absorb backlash.